This invention relates generally to a pressure sensor including electronic circuitry and method which may be used in conjunction with a single or dual variable capacitance pressure transducer. The pressure transducer changes capacitance predictably in response to changes in fluid pressure applied thereto. More particularly, the invention relates to a tuned oscillator circuit and method which supplies a highly accurate output, with minimum frequency jitter, and including a variable capacitance pressure transducer.
The need for highly accurate indications of pressure is particularly acute in the avionics field where measurement of several different pressures is utilized to provide information about aircraft altitude, mach number, air speed, angle of attack, and yaw angle. In both military and commercial aircraft applications such information is necessarily derived from several pressures which must be accurately measured.
The conventional sensor system for measuring each one of such pressures usually includes three components, namely, a pitot tube sensing pressure exterior to the aircraft, a transducer communicating with the pitot-tube for converting sensed pressure into an electrical signal, and a circuit yielding either an analog or digital electrical representation of the pressure communicating to the transducer.
A variable capacitance transducer, as shown in U.S. Pat. No. 3,962,921, to Lips, may be used to convert changes in pressure into changes in capacitance. Circuitry, including the capacitive transducer produces an output signal the frequency of which is dependent on the capacitance of the transducer, and which is in turn dependent upon the fluid pressure applied to the transducer.
One technique for measuring capacitance involves the use of an inductor and capacitor in resonance (L-C circuit). This circuit is naturally a tuned circuit due to the conservative nature of the electrical energy transfer and storage between the two interactive elements. Using an inductor, however, adds undesirable weight to the sensor and limits its use in an aerospace environment.
Another technqiue for measuring capacitance is the single resistor-capacitor (R-C) oscillator producing a voltage triangular waveform across a variable capacitor. The frequency of the triangle waveform is a function of the capacitance value of the variable comparator. A comparator is used to detect the level of capacitor voltage, and to reverse the phase of the triangular wave once the capacitor voltage reaches a given level. However, this triangular waveform oscillator circuit is not tuned, and therefore exhibits a much poorer frequency stability than a tuned circuit. That is, the circuit produces higher levels of frequency jitter than is desired in aerospace applications.
The demands of the aerospace field call for a reduction of size and weight of all aircraft components, including pressure transducers, coupled with improved accuracy and resolution. However, increases in instrument sensitivity bring about undesirable increases in susceptibilty to random interference and jitter. A sensor including electronic circuitry which is light in weight, does not require an inductor, and which can convert the variable capacitance from a pressure transducer into a reliable electronic signal with minimum interference is therefore highly desired in the aviation industry.